I did read this and and now, only after weeks, a possible solution strikes me.Once you remove the built-in FM radio, then these buttons are crippled and using them will cause the Launcher to crash and reset to the default one.
If you use my HWOneKey app, you can select an alternative for a number of apps like the Radio app. So you can assign NavRadio to the "BAND" button/option/call. Even the stock launchers "normally" accept this as the same activity call is used. I did not try yet for those new launchers.
Can you try that?
Secondly: Do you know the Frontapp? That one allows you to put any app in the window area without even referring to the radio app.